The present invention relates to a luminescent pressure sensitive composition for the determination of air pressure patterns on the surface of a body. This invention can be used in various fields of science and technology. It is particularly suitable in determining the air pressure field in aerodynamic investigations.
The most important problem in aerodynamics concerns with the determination of distributed forces acting on a body.
The basic component of these forces originates from the distributed air pressure on a surface of the body. The problem in defining the distributed forces involves the determination of the pressure pattern on the surface of the body. Thus, an appropriate method should be provided to measure air pressure at each point of a surface in experimental researches.
The method uses a surface coating which contains probe molecules (luminophores) that emit luminescence when excited by an appropriate light source. Oxygen molecules in the air interfere with the process of the luminescence and decrease ("quench") the amount of luminescence. As a result, the luminescence of the paint varies as functions of the partial pressure of oxygen. Therefore, the parameters of luminescence (intensity and lifetime) can be related to the static pressure of the air at the coated surface.
A luminescent pressure sensitive composition for the determination of pressure patterns on models in a wind tunnel has been employed. See, for example, J. Kavandi, J. Callis, M. Gouterman, G. Khalil, D. Wright, E. Green, D. Burns, B. McLachian "Luminescent barometry in wind tunnels". Rev. Sci. Instrum., 61, 11, November 1990).
Their composition includes platinum octaethylpoiphyrin (PtOEP) which is dissolved in Genesee Polymers GP-197 dimethylsiloxane polymer solution. The solution is applied on the surface of interest. The 1,1,1-trichloroethane solvent evaporates, and leaves a smooth film. The luminescence of luminophore-PtOEP was excited by an UV lamp with a filter that only allowed light with wavelength above 400 nm to pass through.
Other studies for determining air pressure pattern on a surface have also been conducted. See, for example, (M. J. Morris, J. F. Donovan, J. T. Kegelman, S. D. Schwab, R. L. Levy, R. C. Crites. "Aerodynamic Applications of Pressure-Sensitive Paint". 30th Aerospace Sciences Meeting and Exhibit, Jan. 6-9, 1992/Reno, Nev.
The specific formulation of the composition was not specified. The luminescence of the composition was excited by visible blue light and was detected by a video camera.
The determination of air pressure pattern was carried out by using images of the model taken at wind-off and at wind-on conditions.
The approximation of the luminescence intensity of the composition was modeled by the Stern-Volmer relation: EQU l.sub.o /l=1+K.sub.2 P.sub.02
where l is the intensity of luminescence, l.sub.o is the intensity of luminescence in the absence of oxygen, P.sub.02 is the partial pressure of oxygen, K.sub.q is the Stern-Volmer constant, or EQU l.sub.o /l=1+KP,
where P is the pressure of air.
In the various experiments carried out in a wind tunnel, the results were influenced by model movement and dust contamination during the flow.
The emitted light from a specific point of a surface covered by the composition depended on the amount of excitation light from the light source, but the amount of light received depended on the distance from a specific point to the light source and on the angle of the surface. Furthermore, the intensity of emitted light received by the camera depended on the distance from the camera and on the angle of the surface. Since the model generally has a complex surface, both the distance and the angle to the light source and the camera varies at each point over the surface of the model. Therefore l.sub.o varies at each point over the model. Usually in real experiments l.sub.o is not used as the reference intensity. To determine the pressure at a specific point, the intensity at each point l, at atmospheric pressure P (wind-off condition) is divided by its intensity of the air-flow (wind-on condition). EQU l.sub.i /l=(1+KP)/(1+KP.sub.r)
K- is determined in a calibration pressure chamber.
If the camera and light source are fixed to the tunnel's walls the optical data is recorded at atmospheric pressure at each angle-of-attack.
However, since during the flow the model is subject to movements due to the action of aerodynamic forces and the surface of the model is contaminated by dust particles during a test, the accuracy of the pressure measurements was low.